Such service signals can be of various types, for example command or control signals for apparatus disposed along the line, such as repeaters or amplifiers, or communication signals between the maintenance staff acting at a line point and an intermediate station or terminal of the line itself.
For service signals a restricted transmission band as compared with the band of the communication channels is usually sufficient. An overall transmission speed of 300 Kbit/s, in the case in which service signals are coded on a single digital channel is deemed sufficient, for example.
In an optical-fibre telecommunication system a remedy for signal attenuation along the fibres is necessarily provided by periodically amplifying the signals. The use of optical amplifiers disposed at regular intervals along the transmission line has proved to be convenient.
Such amplifiers, that can be made of optical fibres doped with a fluorescent substance and submitted to optical pumping, are capable of amplifying the signals without converting them to an electric form.
In these lines it is impossible to insert and extract signals into and from the fibre along which they are transmitted by means of known electronic apparatus, because signals are available in the optical form even close to the amplifiers.
U.S. Pat. No. 5,113,459 in the name of the assingee of this application describes an optical telecommunication system optionally provided with optical amplifiers along the line, in which insertion and extraction of the service channel takes place by dichroic couplers.
Also provided in this system are receiving and emitting units connected to the dichroic couplers, adapted to receive optical service signals from the line, convert them to electric signals and electronically amplify them, and to receive the amplified electric signals, convert them to optical signals at the wavelength of the service channel and send them to the line, respectively.
In order to carry out separation between the signals by means of dichroic couplers, the wavelength of the service channel has been selected considerably different from that of the communication channels. In addition, for minimizing attenuation for the service channel, this wavelength has been selected substantially coincident with or to a small distance from a minimum of the spectral attenuation curve of light in the optical fibre.
In the case in which the telecommunication signal wavelength is substantially included between 1500 and 1600 nm (the so-called "third window" for silica-based optical fibres) and the service channel wavelength is included in the so-called "second window", for silica-based optical fibres located in the vicinity of the relative attenuation minimum at about 1300 nm, the attenuation to which the service signals are submitted is much greater than that relating to the communication channels.
In fact, at the second window wavelengths, the attenuation coefficient for silica-glass optical fibres usually in use has a value typically included between 0.37 and 0.41 dB/km, against a typical value of about 0.2 dB/km for wavelengths within the third window.
The length of the line portion included between two amplifiers or between one of the end stations and one of the amplifiers is given by the maximum acceptable attenuation at the wavelengths of the communication channels, in turn linked to the maximum available gain at those wavelengths.
With the optical amplifiers presently in use this maximum gain is about 25-30 dB.
The overall attenuation value at the wavelength of the service channel along the portion between two amplifiers may therefore reach values higher than 50 dB, for example.
In order to generate the radiation to be used for transmission of the service channel, the use of semiconductor lasers is provided. Semiconductor lasers with emission at the wavelength of the second window have a typical output power of about 1 mW (0 dBm). Lasers having a greater output power are undesirable due to their high costs.
Taking into account the reduction of the laser output power in time and aging of the passive optical components along the transmission line, a further power reduction at the receiver of about 8 dB can be expected.
The problem exists therefore of transmitting digital signals, in particular service signals, along an optical communication line and receiving them with a sufficiently low error rate, in the presence of a limited power at the receiver.